The present invention relates generally to medical stents (hereinafter “stents”) and relates more particularly to stent delivery systems.
A stent is a tubular endoprosthesis placed within a body lumen to maintain its patency. For example, a stent may be used to maintain the patency of an esophagus or other passageway occluded by a tumor or of a blood vessel constricted by plaque. Most stents fall into one of the following two classes: (i) balloon-expandable stents; and (ii) self-expandable stents. Generally speaking, a stent delivery system includes the stent and some means for positioning and fixing the stent in place within a body lumen.
A balloon-expandable stent is typically a ductile metal tube. The delivery system for such a stent typically includes an inflatable balloon secured to the distal end of a catheter, the balloon-expandable stent being secured to the catheter over the inflatable balloon. In use, the catheter is introduced to a desired site within a body lumen, and the balloon is expanded until the stent positioned thereover is inelastically expanded to its desired size against the walls of the lumen. The balloon is then deflated, and the catheter is withdrawn from the site, thereby leaving the expanded stent in place against the walls of the lumen. An example of a balloon-expandable stent and delivery system therefor is disclosed in U.S. Pat. No. 4,922,905, inventor Strecker, issued May 1990, the disclosure of which is incorporated herein by reference.
A self-expandable stent is typically an elastic tube that self-expands after having been compacted. Illustrative examples of self-expandable stents are disclosed in the following documents, all of which are incorporated herein by reference: U.S. Pat. No. 5,876,445, inventors Andersen et al., issued Mar. 2, 1999; U.S. Pat. No. 5,366,504, inventors Andersen et al., issued Nov. 22, 1994; U.S. Pat. No. 5,234,457, inventor Andersen, issued Aug. 10, 1993; U.S. Pat. No. 5,061,275, inventors Wallstén et al., issued Oct. 29, 1991; Watkinson et al., “The Role of Self-Expanding Metallic Endoprostheses in Esophageal Strictures,” Seminars in Interventional Radiology, 13(1): 17–26 (March 1996); and Strecker et al., “Nitinol Esophageal Stents: New Designs and Clinical Indications,” Cadiovasc. Intervent. Radiol., 19:15–20 (1996).
The delivery system for a self-expandable stent typically comprises a catheter and a restraint for temporarily holding the stent in a compressed state at the distal end of the catheter. In use, the catheter is introduced to a desired site within a body lumen, and the restraint is removed, thereby allowing the stent to expand by its own elastic restoring force against the walls of the lumen. One of the more common self-expandable stent delivery systems of the above-described type comprises a coaxial tube assembly. More specifically, said delivery system comprises an inner catheter and an outer catheter, said outer catheter being coaxial with said inner catheter. The inner and outer catheters are appropriately sized so that a stent inserted over the distal end of the inner catheter is maintained in a compressed state by the inner surface of the outer catheter. In use, the assembly is introduced to a desired site, and the outer catheter is axially retracted relative to the inner catheter, thereby allowing the stent to self-expand off the inner catheter and against the walls of the lumen.
Illustrative examples of the aforementioned type of delivery system include the following U.S. patents, all of which are incorporated herein by reference: U.S. Pat. No. 5,484,444, inventors Braunschweiler et al., issued Jan. 16, 1996; U.S. Pat. No. 5,026,377, inventors Burton et al., issued Jun. 25, 1991; U.S. Pat. No. 4,990,151, inventor Wallstén, issued Feb. 5, 1991; and U.S. Pat. No. 4,732,152, inventors Wallstén et al., issued Mar. 22, 1988. A commercial embodiment of the above-described delivery system is the UNISTEP PLUS™ delivery system (Boston Scientific Corporation, Natick, Mass.).
Although the above-described coaxial tube delivery system is well-suited for many types of self-expandable stents, such a delivery system is not particularly well-suited for those self-expandable stents that have limited axial strength, such as the Strecker stent—a knitted nitinol wire stent disclosed in U.S. Pat. No 5,366,504 and commercially available from Boston Scientific Corporation, Natick, Mass. This is because such stents are often unable to resist becoming axially compressed during assembly of the coaxial tube delivery system, when the distal end of the outer catheter must be drawn across the stent, and/or during deployment of the coaxial tube delivery system, when the distal end of the outer catheter must be withdrawn across the stent.
Consequently, alternative delivery systems have been devised for use with the Strecker stent and with other like stents of limited axial strength. One such system, which is exemplified by the ULTRAFLEX™ esophageal stent system (Boston Scientific Corporation, Natick, Mass.), comprises a stent of the aforementioned type, said stent being mounted on an inner catheter and being encased in a pharmaceutical grade dissolvable gelatin to maintain the stent in a compressed state. An outer catheter surrounds the gelatin-encased stent. In use, the system is inserted into a patient's esophagus and the outer catheter is retracted. Moisture present in the esophagus causes the restraining gelatin to dissolve, thereby allowing the stent to expand.
Another delivery system devised for use with the Strecker stent and like stents of limited axial strength is disclosed in U.S. Pat. No. 5,405,378, inventor Strecker, which issued Apr. 11, 1995, and which is incorporated herein by reference. Said delivery system, which does not include an outer catheter, uses a crocheted suture cord to compress the stent against a catheter core. The crocheted cord is connected to a finger ring at the proximal end of the delivery system. Retraction of the finger ring unravels the cord in a circular manner down the length of the stent, gradually deploying the stent. A commercial embodiment of the aforementioned delivery system is the COVERED ULTRAFLEX™ esophageal stent system (Boston Scientific Corporation, Natick, Mass.).
Unfortunately, the manufacture of the aforementioned crocheted cord delivery system is typically performed manually and can be quite taxing physically as a great deal of strength is required to tightly stretch the stent down on the catheter while crocheting the cord around the stent. Another problem with the foregoing system is that, during deployment, the crocheted cord does not always unravel correctly and completely.